Crawler belts made of rubber for a crawler unit have been developed. As disclosed in patent documents 1 to 3 listed below, the crawler belts have a multitude of tread lugs arranged at intervals on outer peripheries thereof. The tread lugs catch on steps or bumps on the ground, thereby allowing the crawler unit to move over the steps or bumps.    Patent Document 1: Japanese Patent Application Laid-Open No. H6-156333    Patent Document 2: Japanese Patent Application Laid-Open No. H6-199253    Patent Document 3: Japanese Patent Application Laid-Open No. H6-329057